P0420/P0430 codes are not necessarily coincidental to restricted flow.
Most dead cats flow just fine. It's just that the catalyst is contaminated/depleted and isn't performing any useful activity.
Plugged cats usually cause other problems, not efficiency codes.
Our 98 2.5 ran perfectly fine with high fuel efficiency with the efficiency codes set for two months while I was dinking with it and then waiting for the dealer to replace the pre-cats.
Remember, the downstream O2 sensors are only used for monitoring pollution controls, nothing else.
Steve
